High temperature membrane electrode assembly with high power density and corresponding method of making

ABSTRACT

A membrane electrode assembly (MEA) with enhanced current density or power density is fabricated using high temperature (HT) proton exchange membrane (PEM). The MEA can be utilized in high temperature PEM fuel cell applications. More specifically, the MEA is modified with the addition of one or more of selected materials to its catalyst layer to enhance the rates of the fuel cell reactions and thus attain dramatic increases of the power output of the MEA in the fuel cell. The MEA has application to other electro-chemical devices, including an electrolyzer, a compressor, or a generator, purifier, and concentrator of hydrogen and oxygen using HT PEM MEAs.

RELATED APPLICATION

This application claims priority to, and the benefit of, co-pending U.S.Provisional Application No. 61/320,040, filed Apr. 1, 2010, for allsubject matter common to both applications. The disclosure of saidprovisional application is hereby incorporated by reference in itsentirety.

FIELD OF THE INVENTION

The present invention relates to enhancement of current density or powerdensity of a membrane electrode assembly (MEA), and more particularly tomodification of an MEA with the addition of one or more of selectedmaterials to its catalyst layer to enhance the rates of the fuel cellreactions and thus attain increases of the power output of the MEA inthe fuel cell.

BACKGROUND OF THE INVENTION

PEM fuel cells are well known in the art; as a power generation device,they convert chemical energy of fuels to electrical energy without theircombustion and therefore without any environmental emissions. A PEM fuelcell like any electro-chemical cell of the stated categories, is formedof an anode and a cathode interposed by a layer of an electrolytematerial for ionic conduction.

Embodiments of the conventional electro-chemical cell also includehardware components, e.g., plates, for reactant flow separation, currentcollection, compression and cooling (or heating). A separator plateprovides multiple functions: (a) distributes reactant flow at the anodeor cathode, (b) collects electrical current from operating anode/cathodesurface and (c) prevents mixing or cross-over of the anode and cathodereactants in single cells. An assembly of two or more of these singlecells is called a stack of the electro-chemical device. The number ofsingle cells in a fuel cell stack is generally selected based on adesired voltage of the resulting stack. Conventionally desired voltagesinclude 12 volts, 24 volts, 36 volts, 120 volts, and the like. Forconvenient assembly and/or dis-assembly of a fuel cell stack with largevoltage or power output, multiple sub-stacks or modules, are combined toform the stack. The modules represent stacks of single cells in somenumber less than what ultimately results in the completed stack, as iswell understood by those of skill in the art. When the stack forms a PEMfuel cell, such a module is often referred to as a PEM stack.

In the membrane electrode assembly (MEA) fuel (e.g., hydrogen) andoxidant (e.g., oxygen or air) react at the interfacial structures of theMEA to generate electrical power. Current HT MEAs are fabricated using aspecific type of PEM material that contains phosphoric acid in itspolymer matrix structure. The host matrix in the high temperature PEMmaterial is thus a high temperature polymer material in whichconcentrated phosphoric acid is infused, and which is responsible forthe proton conduction of the high temperature (e.g., 120° C. to 200° C.)PEM fuel cells. These HT PEM materials thus allow the fuel cell tooperate at temperatures typical of phosphoric acid fuel cell (PAFC)operation (e.g., up to 200° C.) that are much higher than that ofconventional low temperature PEM fuel cell operation (about 80° C.),which is typically well below 100° C.

While high temperature operation brings in a number of benefits,including but not limited to carbon monoxide (CO) tolerance, usefulquality heat, fuel cell system simplification, and the like, the current(or power density per unit area) of the MEA is drastically reduced ascompared to low temperature PEM MEAs, such as, e.g., MEAs made withNafion® by E. I. du Pont de Nemours and Company. The power density perunit area of the MEA is reduced primarily due to intrinsic slow kineticsof oxygen reduction at the catalyst (typically Platinum)-phosphoric acidinterface. More specifically, historical MEA technology development wastargeted to the conventional low temperature PEM fuel cells in which thePEM material was Nafion® or equivalent perfluoro sulfonic acid ionomers.Addition of these ionomer materials in the interfacial structure of MEAshas been utilized to enhance the power output of MEAs (e.g., S.srinivasan et al., J Power Sources, 22,359, 1988/29,367, 1990 and MahlonS. Wilson et al., J. Electrochem. Soc., Vol. 139 No. 2, 1992). In U.S.Pat. No. 5,272,017, a proton conducting material was used to make aslurry of carbon supported catalyst particles; the slurry was applied toopposed surfaces of the PEM, which was then hot-pressed to embed atleast a portion of the particles into the membrane. In a similar examplein U.S. Pat. No. 5,882,810, the active layer of the MEA containedcatalytically-active particles and an ionomer with lower equivalentweight than that of the PEM material itself. In yet another example, inU.S. Pat. No. 6,287,717 B1, an electrode comprising catalytically activemetal particles and an ionically conductive polymer (ionomer) was bondedto an ionically conductive polymer membrane to form anelectrode-membrane interface.

Current technology for high temperature MEAs containing phosphoric acidin the polymer matrix does not include addition of any ionomer in thecatalyst layer. However, different acidic materials includingperfluorinated sulfonic acid have been used as additives in the body ofhigh temperature PEM membranes, particularly to enhance their protonconductivity.

SUMMARY

There is a need for an MEA having significantly enhanced power densityper unit area of the MEA relative to current high temperature PEM MEAtechnology. The present invention is directed toward further solutionsto address this need, in addition to having other desirablecharacteristics.

In accordance with one example embodiment of the present invention, anMEA is formed with enhanced kinetics of the cathode reaction at the hightemperature PEM-catalyst interface of a fuel cell MEA. Presence of anionomer or a perfluoroacid in the body of the membrane would be expectedto have some exposure to the interfacial area, and thus may havepositive effects on the reaction kinetics due to its high oxygensolubility and weak anion adsorption on catalyst surface. However, toavail the fullest benefit of an ionomer or other additive material(s),the present invention makes use of such additive(s) in the catalystlayer of the MEA. In accordance with one example embodiment of thepresent invention, the enhanced kinetics is accomplished by placing aselected additive material (or materials) at the PEM-catalyst layerinterface (the interface between the PEM and the anode and cathodeelectrodes). The additive material (or materials) creates a morereactive interface with the catalyst (Pt or Pt-alloy) for faster oxygenreduction kinetics. The additive material (or materials) can be solid orliquid but must be stable at the desired fuel cell operatingtemperatures (e.g., 120° C. to 200° C.). The amount of the additivematerial in the catalyst layer is in the range of 1% to 15% of thecatalyst weight.

In accordance with one embodiment of the present invention, the additivematerial(s) may alternatively have a nano-structural characteristicand/or wetting behavior with respect to phosphoric acid present in thehigh temperature PEM matrix; wetting enlarges the three-phase interfacearea where the oxygen reduction takes place. This type of material mustalso be highly stable at temperatures of 120° C. to 200° C. andpreferably be available at low-cost. Example additive materials fittingthese requirements include, but are not limited to, selected acids,oxides, and oxyacids, acid salts including phosphates, sulfates,carbides, silicates, selected fluoropolymers, ionomers (including, e.g.,Nafion® or equivalent PEM materials), and the like, with further detailsand additional example materials included herein as further describedbelow.

In accordance with another aspect of the present invention, the hightemperature PEM MEA is fabricated with a modified interfacial structure.The MEA is fabricated by co-depositing the additive material(s) with thecatalyst particles at the electrode surface using any suitable coatingtechniques (e.g., spray-coating or printing using the catalyst ink).Those of skill in the art will appreciate that the present invention isby no means limited to the specific coating techniques described herein.Rather, any coating technique allowing the co-deposit of the additivematerial(s) with catalyst particles can be utilized, and is anticipatedfor use in accordance with the present invention.

BRIEF DESCRIPTION OF THE FIGURES

These and other characteristics of the present invention will be morefully understood by reference to the following detailed description inconjunction with the attached drawings, in which:

FIG. 1 is an exploded view of a fuel cell stack, according to one aspectof the present invention;

FIG. 2 is a cross-sectional illustration of an MEA in accordance withone aspect of the present invention; and

FIG. 3 is a diagrammatic illustration of an MEA interface whereelectro-chemical reactions occur during the operation of the fuel cellstack in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION

An illustrative embodiment of the present invention relates to an MEAhaving significantly enhanced power density per unit area of the MEArelative to known high temperature PEM MEAs, and a corresponding methodof making. More specifically, current high temperature PEM MEAs have apower density per unit area of, for example, 120-150 mW/cm² at 0.67Vwith ambient pressure H₂/Air operating at 180° C. The MEA of the presentinvention has a power density per unit area of about 200 mW/cm² at 0.67Vwith ambient pressure H₂/Air operating at 180° C. This is achieved byco-depositing with the catalyst particles an additive (or additives)having desired reactionary properties as described herein into a layerwhen forming the MEA. The additive material(s) can also be anano-structural material of an organic or inorganic acid, or acid salt,phosphates, sulfates, carbides, silicates, and the like, in addition toother materials further described herein, and their equivalents.

FIG. 1 is an expanded view of a fuel cell stack 20 in accordance withone example embodiment of the present invention. First and secondcompression plates 22, 24 form the top and bottom plates. Adjacent thecompression plates 22, 24 are current collector plates 26, 28. Aninsulator laminate can be provided between the compression plates 22, 24and the current collector plates 26, 28 as would be well understood byone of skill in the art. Adjacent the collector plates are a pluralityof support plates and MEAs. As shown in the figure, there is a firstsupport plate, 30, a second support plate 32, and a third support plate34. Sandwiched between each support plate is an MEA 10. As shown in thefigure, there is a first MEA 10 a, and a second MEA 10 b. The supportplate 30, 32, 34 includes a first seal 50 a and a second seal 50 b, eachpositioned on opposing sides of a supporting plate 40. The first andsecond seals 50 a, 50 b, are adhered to the supporting plate 40 to formeach of the first, second, and third support plates 30, 32, 34. The seal50 a, 50 b is affixed on each side of the support plate 30, 32, 34, andis configured for sealing against the MEA 10 (10 a, 10 b), or thecurrent collector plates 26, 28 with compression. Those of skill in theart will appreciate that other configurations of fuel cell stacks exist.As such, the present invention is by no means limited to theillustrative embodiment shown in FIG. 1. Rather, any fuel cell stackmaking use of an MEA 10 (10 a, 10 b) as described herein is anticipatedfor use in accordance with the present invention.

The description provided herein provides an illustrative example of amembrane electrode assembly having an additive (or additives)co-deposited with a catalyst thereon, and corresponding method ofmanufacture, according to the present invention. Although the presentinvention will be described with reference to the example embodiment, itshould be understood that many alternative forms can embody the presentinvention. One of skill in the art will additionally appreciatedifferent ways to alter the parameters of the embodiments disclosed,such as the size, shape, or type of elements or materials, in a mannerstill in keeping with the spirit and scope of the present invention.

Two conventional methodologies for membrane electrode assembly (MEA)fabrication are: (a) application of the catalyst particles on aconductive support surface or electrode and then bonding the catalyzedelectrode with PEM surface to create the membrane-electrode interface,and (b) application of the catalyst particles on the PEM surfacedirectly to fabricate the catalyst-coated membrane (CCM) and thenattaching or bonding the conducting electrode material (electrode withgas diffusion layer, GDL) with the CCM surface to form the MEA.

With respect to the present invention, the process of catalyst-coatedmembrane (CCM) fabrication is relatively more cumbersome usingphosphoric acid infused high temperature proton exchange membrane (HTPEM). As such, manufacture of the MEA in accordance with the presentinvention can be done in a more similar fashion to the first methoddescribed above, namely, application of catalyst particles on aconductive surface or electrode, which is then bonded to the protonexchange membrane (PEM) surface to create the MEA.

However, the method of manufacture in accordance with the presentinvention differs from the conventional method. In accordance with thepresent invention, an additive (or additives) is co-deposited with thecatalyst particles onto the electrode surface using any suitable coatingtechniques (e.g., spray-coating or printing using the catalyst ink). Oneof skill in the art will appreciate that there are multiple differentcoating technologies that can be used to apply the additive and catalystparticles to the electrode. As such, the present invention is by nomeans limited only to those described herein. Rather, it is theco-depositing of the additive(s) with the catalyst that is relevant, andany coating method capable of such co-depositing methodology isanticipated for use with the present invention.

The result of the manufacturing process is illustrated in FIG. 2, whichshows a cross-sectional view of an MEA 10, in accordance with oneembodiment of the present invention. The MEA 10 is formed of a protonexchange membrane (PEM) 70. An electrode support layer 60, typicallyformed of carbon-based material (e.g., carbon fabric or paper), isplaced against the membrane 70 and typically bonded using heat in aprocess known to those of skill in the art. Between the membrane 70 andthe electrode support layer 60, a layer of carbon 62 may optionally beadded. The electrode support layer 60 and, if added, the carbon layer,are often referred to as a gas diffusion layer. In addition, a catalystlayer 64 is provided, which may contain Teflon® (made by E. I. du Pontde Nemours and Company) as a binder material. In accordance with thepresent invention, the catalyst layer 64 includes a catalyst and anadditive (or additives) in addition to the binder material. Asrepresentative examples, the materials forming these layers include acarbon-based material for the electrode support layer 60, phosphoricacid infused high temperature proton exchange membrane 70, and a mixtureof platinum catalyst and additive(s) for the catalyst layer 64. However,as further described herein, and as would be understood by those ofskill in the art, the specific materials used for each layer of the MEA10 can vary depending on preferences and desired characteristics of theresulting MEA 10. As such, the present invention is by no means limitedto these specific materials described in the representative example.

More specifically with regard to the additive material(s), the additivematerial(s) creates a more reactive interface with the Pt or Pt-alloycatalyst for faster oxygen reduction kinetics because of one or more ofthe following effects: (a) acid molecules with specific adsorption muchlower than that of phosphoric acid will leave more reaction sites activefor oxygen reduction (phosphoric acid adsorbs strongly and blocks mostof the reaction sites on Pt or Pt-alloy catalyst particles); (b)presence of fluorinated polymers, ionomers, and acids in the immediatevicinity of the catalyst sites enhances oxygen concentration locally toaccelerate its reaction rate at the interface; and (c) optimal wettingproperty of the additive material(s) increases the interfacial area foroxygen reduction reaction provided flooding of the interface due toexcess wetting is avoided. The additive material(s) can includematerial(s) with desired reactionary properties with the catalyticinterface in contact with the acid found in the MEA (conventionally,phosphoric acid) which can be solid or liquid but must be stable at thedesired fuel cell operating temperatures (e.g., between about 120° C.and 200° C.).

FIG. 3 is a diagrammatic illustration representing the conceptualimplementation of the above-described chemistry, in accordance withaspects of the present invention. As illustrated, layer A is theelectrode support layer 60, layer B (B₁, B₂) is the catalyst layer 64,and layer C is the PEM membrane 70. The magnified portion of thecatalyst layer 64 (layer B (B₁, B₂)) shows a catalyst support particleO, a catalyst particle P, and an additive material particle Q. Thecatalyst support particle O can be, for example, a high surface areacarbon/graphite powder. The catalyst particle P can be, for example, aplatinum or platinum alloy. The additive material particle Q can be, forexample, one or more of the additive materials described herein.

In the electro-chemical process relied upon by the present invention,the following reactions occur at the interface shown in FIG. 3.

Anode: H₂→2H⁺+2e ⁻  (1)

Cathode: ½O₂+2H⁺+2e ⁻→½H₂O  (2)

Total Cell: H₂+½O₂→H₂O  (3)

As shown above, a total cell reaction (3) represents a cell reaction inan operating H₂/O₂ (Air) fuel cell while individual electrode reactionsat the anode and the cathode are described in an anode reaction (1) anda cathode reaction (2), respectively. The anode reaction (1) takes placeat the interface of the anode-PEM interface (B₁-C in FIG. 3), where theH₂ molecules dissociate to produce protons and electrons. When the anodeand the cathode are electrically connected, the electrons flow throughthe external circuit, while the protons migrate to the cathode throughionic conduction. The O₂ molecules (in air) fed at the cathode meet themigrated protons and electrons at the interface of the cathode-PEMinterface (C-B₂ in FIG. 3), where the cathode reaction (2) takes placeproducing H₂O molecules.

In accordance with the present invention, it should be noted that theanode reaction rate is faster than the cathode reaction by orders ofmagnitude. For example, at a practical operating voltage (0.67 V/cell)of HT PEM fuel cell, the overvoltage (voltage loss: theoretical voltageminus the actual operating voltage) at the anode is typically <20 mV;whereas the cathode overvoltage can be >400 mV. It is established thatmost of this overvoltage is activation overvoltage, which means thevoltage loss is due to the very slow rate of the cathode reaction.Accordingly, the present invention modifies the cathode-PEM interface toenhance the rate of oxygen reduction reaction (ORR) represented in thecathode reaction (2). This interface is further illustrated in themagnified portion of FIG. 3, where the reaction site (R) is located atthe three-phase (catalyst-electrolyte-oxygen) interface, the electrolytephase being the phosphoric acid-infused HT PEM. While intimate contactbetween the catalyst and the HT PEM is essential for the creation of thereactive interface (R) at their immediate contact area, extension ofsuch reactive surface area in the depth of the catalyst layernecessitates penetration of the electrolyte phase into the catalyststructure, but without blocking the access of oxygen (air) to thereaction site (R). The optimal wetting characteristic of the catalystlayer with phosphoric acid is thus a critical requirement for optimaloperation in accordance with the present invention.

As previously stated, conventional HT PEM materials have concentrated(85%-100%) phosphoric acid infused in a polymer matrix. Therefore, thecathode-PEM interface (C-B₂ in FIG. 3) in a HT PEM MEA essentiallycontains catalyst (supported or unsupported Pt or Pt-alloy) particles onthe electrode side (B (B₁, B₂)) and phosphoric acid on the electrolyteside (C). A phosphoric acid molecule in equilibrium with itssuccessively dissociated anionic species is shown in FIG. 3 and ReactionScheme 1 herein. Strong adsorption of these species and low solubilityof oxygen in phosphoric acid are considered contributing significantfactors for the slow rate of ORR at the Pt-phosphoric acid interface.The addition of trifluoromethanesulfonic acid in 85% phosphoric acidresults in significant enhancement of the ORR rates in the mixed acid ofdifferent composition (e.g., Enayetullah at al., J. Applied Electrochem,18, 763, 1988); the additive acid, in addition to being weakly adsorbingon the Pt surface, has much a higher solubility of oxygen within.

In accordance with the present invention, judicious selection ofadditive materials to the catalyst layer is based on their weakeradsorption (on Pt) and higher oxygen solubility as compared to those inconcentrated phosphoric acid. The wetting characteristic of the additiveby phosphoric acid is another factor that can affect the ORR rate, morethe wetability of the catalyst layer favors the creation of a largerinterfacial area. However, excessive wetting of the catalyst layercauses a flooding situation, preventing access of oxygen molecules (fromair) to the reaction site. The addition of Teflon particles as anon-wetting agent in the catalyst phase is necessary to minimizeflooding of the interface. The present invention creates an optimalinterfacial structure with the additive acidic materials (with highoxygen solubility and week specific adsorption) in immediate contactwith the catalyst particles (reaction sites (R)) and the phosphoricacid, optimally wetting the additive/catalyst surface for enlargedreactive interfacial area.

In accordance with the present invention, high performance MEAs for hightemperature PEM fuel cells operating at temperatures between about 120°C. and 200° C. include concentrated phosphoric acid infused in a hightemperature polymer matrix. The PEM-electrode interface structure isaltered to include an additive material (or materials) together with acatalyst. The catalyst layer is typically fine particles of Platinum(Pt), or the Pt particles supported on high surface area supportmaterials. In accordance with the present invention, the catalyst layeris modified by inclusion of the additive material(s). The additivematerial(s) can be in solid or liquid form, and be a particle (orparticles) having desired reactionary properties with the acid in the HTPEM, and which is stable at temperatures of HT PEM fuel cell operation(e.g., 120° C. to 200° C.). Illustrative examples of such additivematerials include, but are not limited to:

-   -   Fluoropolymers: Perfluoro ionomers, e.g., Nafion® and other        equivalents    -   Phosphates: Zr₃(PO₄)₄, Zr(HPO₄)₂, HZr₂(PO₄)₃, Ti(HPO₄)₂, KH₂PO₄,        CsH₂PO₄, MgHPO₄, HSbP₂O₈, HSb₃P₂O₈, H₅Sb₅P₂O₂₀, etc.    -   Sulfates: MHSO₄ (M: Li, Na, K, Rb, Cs & NH₄)    -   Polyacids: H₃PW₁₂O₄₀.nH₂O (n=21-29), H₃SiW₁₂O₄₀.nH₂O (n=21-29),        H_(X)WO₃, HSbWO₆, HNbO₃, HTiNbO₅, H₅Ti₄O₉, HSbO₃, H₂MoO₄, etc.    -   Selenites and Arsenites: M₃H(SeO₄)₂ (M: Cs, Rb and NH4),        KH₂AsO₄, UO₂AsO₄, etc.    -   Phosphides: ZrP, TiP, HfP, etc.    -   Oxides: Al₂O₃, Sb₂O₅, SnO₂, ZrO₂, etc.    -   Silicates: Zeolites, H-Natrolites, H-Mordenites, Clays, etc.    -   Superacids: Sb₂F₅, Fluorosulfonic acids, sulfonic acids, etc.

The additive material(s) can be a nano-structural material of an organicor inorganic acid, or acid salt with suitable wetting property incontact with phosphoric acid, some examples of which are included above,and others of which would be understood and identifiable by those ofskill in the art, as such the above list is in no way limiting to thematerials available for use in accordance with the present invention.The amount of the additive material in the catalyst layer is in therange of 1 to 15% of the catalyst weight.

With the inclusion of the additive(s), in accordance with the presentinvention, the resulting high temperature PEM MEAs have a power densityper unit area of, for example, 180-300 mW/cm² at 0.67V with ambientpressure H₂/Air operating at 180° C., as compared with a conventional HTPEM MEA having a power density per unit area of 120-150 mW/cm² at 0.67Vwith ambient pressure H₂/Air operating at 180° C.

Numerous modifications and alternative embodiments of the presentinvention will be apparent to those skilled in the art in view of theforegoing description. Accordingly, this description is to be construedas illustrative only and is for the purpose of teaching those skilled inthe art the best mode for carrying out the present invention. Details ofthe structure may vary substantially without departing from the spiritof the present invention, and exclusive use of all modifications thatcome within the scope of the appended claims is reserved. It is intendedthat the present invention be limited only to the extent required by theappended claims and the applicable rules of law.

It is also to be understood that the following claims are to cover allgeneric and specific features of the invention described herein, and allstatements of the scope of the invention which, as a matter of language,might be said to fall therebetween.

1. A membrane electrode assembly, comprising a high temperature protonexchange membrane disposed between an anode electrode and a cathodeelectrode; wherein a catalyst and at least one additive are disposed ina layer applied at an interface between the membrane and each of theanode and cathode electrodes.
 2. The assembly of claim 1, wherein thecatalyst comprises platinum, or platinum-alloy particles.
 3. Theassembly of claim 1, wherein the catalyst particles are unsupported orsupported on a high surface area conductive powder material.
 4. Theassembly of claim 1, wherein the at least one additive comprises anorganic or inorganic acid material, acid salt, fluoropolymer,perfluoroacid electrolyte, or an ionomer in an amount of between about1% and 15% of a weight of the catalyst at the interface.
 5. The assemblyof claim 1, wherein the at least one additive comprises a materialhaving favorable wetting and reactionary properties with the catalyst incontact with phosphoric acid.
 6. The assembly of claim 1, wherein theresulting assembly has a power density per unit area of between about180 and 300 mW/cm² at 0.67V with ambient pressure H₂/Air operating at180° C.
 7. The assembly of claim 1, wherein the resulting assembly has apower density per unit area of about 200 mW/cm² 0.67V with ambientpressure H₂/Air operating at 180° C.
 8. A method of manufacturing amembrane electrode assembly, comprising: co-depositing a catalyst and anadditive layer to an interfacing surface of a first electrode and aninterfacing surface of a second electrode; and bonding or attaching thefirst electrode to a first side of a high temperature proton exchangemembrane along the interfacing surface of the first electrode andbonding or attaching the second electrode to a second side of the hightemperature proton exchange membrane along the interfacing surface ofthe second electrode.
 9. The method of claim 8, wherein the catalystcomprises supported or unsupported platinum.
 10. The method of claim 8,wherein the catalyst comprises platinum containing alloy particles. 11.The method of claim 8, wherein the additive layer comprises at least oneadditive formed of an acidic material, an ionomer, or both.
 12. Themethod of claim 8, wherein the additive layer comprises at least onematerial having favorable wetting and reactionary properties with thecatalyst in contact with phosphoric acid.
 13. The method of claim 8,wherein the resulting assembly has a power density per unit area ofbetween about 180 and 300 mW/cm² at 0.67V with ambient pressure H₂/Airoperating at 180° C.
 14. The method of claim 8, wherein the resultingassembly has a power density per unit area of about 200 mW/cm² at 0.67Vwith ambient pressure H₂/Air operating at 180° C.